The present invention pertains to offset printing blankets used in offset printing operations, and more particularly to a method of treating said blankets so as to greatly retard or eliminate the rusting of offset printing blanket cylinders, caused by repeated exposure of the cylinders to the water which is used in conventional offset printing operations.
In a conventional offset printing operation, printing plates carrying photographic exposures showing the images to be printed, in correct form, are mounted upon plate cylinders. Offset printing blankets, mounted on separate blanket cylinders, are used as an image transfer means. During the printing operation the printing blankets, attached to rotating blanket cylinders, each roll against a plate cylinder, picking up reversed images from the printing plate or plates. These reversed images are then transferred to paper with a second reversal, resulting in correct final images, when the paper is passed between two blanket cylinders, when printing on both sides of the page, or between a blanket cylinder and another cylinder when printing on one side only, which presses the paper against the blanket.
Water is applied to the printing plates in the conventional offset printing operation, because the water tends to keep the printing ink from sticking to non-exposed areas of the printing plates. The presence of water in the press obviously presents a potential for corrosion of steel parts of the press, such as the plate cylinders and blanket cylinders. Although the cylinders are nickel coated, and underlying steel becomes exposed through wear, corrosion or rust and the physical damage caused by passing a foreign object between the press cylinders during press operation. The nickel coating is also seen to abrade away by the movement of the blanket at the unabutted edge of each blanket. Each blanket may have an unabutted edge, since there are sometimes two blankets per blanket cylinder, each covering the full circumference and half the cylinder width. Therefore, during printing operations using both blanket sections on the blanket cylinder, there is greater movement at the free edges of the blankets, at the cylinder edges, than at the center of the cylinder where the two blankets meet. This movement varies from blanket to blanket due to the human factor, since each blanket is hand fastened by a pressman.
The printing plate are ordinarily removed from the plate cylinders after each press run, at least where new material is to be printed in the next run, i.e. daily in the case of a newspaper, allowing cleaning and drying of the plate cylinder surfaces between press runs. Since the plate cylinders are thus accessible for cleaning between press runs, corrosion of the plate cylinders from exposure to the water has ordinarily not been a serious problem.
However, the situation is different as to the blanket cylinders. The printing blankets are only changed when damaged or worn out of tolerance, since it is possible, by washing them down with a solvent, to remove any residual ink remaining after completion of the press run; the residual ink may be minimal in any case, assuming that the inking has been adjusted for the number of pages to be produced in the press run. Since the printing blankets typically remain on the blanket cylinders for an extended period of time, and since the printing blankets roll against the printing plates, they are also exposed to the water, and the water tends to seep under the edges of the printing blankets, onto the blanket cylinders, particularly near the edges of the blanket. Therefore a serious corrosion problem has resulted for nickel plates or other metal plated steel blanket cylinders used in offset printing operations. Marked corrosion of the blanket cylinders is often observed after significant use. This corrosion tends to be concentrated near the edges of the blankets, indicating that it is caused by electrochemical action promoted by the water seeping under the edges of the printing blankets, and/or by mechanical damage, from removal of nickel plating due to contact with foreign material in printing operations. Although some blanket cylinders have been made from stainless steel in recent years, in an effort to deal with this problem, these are much more expensive than ordinary steel cylinders, and have not been found to be fully satisfactory in eliminating such corrosion.
The approach of applicant's invention is simple, and much less expensive one, based on the concept of adding a sacrificial metal, more chemically active than the iron contained in the steel of the blanket cylinders, to the surfaces of the printing blankets which are in contact with the surfaces of the blanket cylinders. Such a sacrificial metal can take the place of the iron atoms in electrochemical actions induced by the water. Of the various possible sacrificial metals which might be used for this purpose, applicant selected zinc as the only one which was practicable under actual working conditions. Thus the zinc additive of the printing blankets corrodes, in lieu of the steel of the blanket cylinders. Since the printing blankets are replaced after a flew months use anyway, this presents no practical disadvantage for the printing operation. Moreover, zinc is quite inexpensive, allowing a significant cost saving as compared with the use of stainless steel blanket cylinders, which have in any case not been fully effective.